The long-term goals of this PPG are to understand the pathogenesis of myeloid leukemias and myeloproliferative disorders (MPDs) and use this information to develop novel and effective therapies. It is proposed that the ideal targets for therapy are the protein products of the oncogenes that cause acute or chronic myeloid diseases, and this proposal will continue to focus on tyrosine kinases. In the last cycle of this grant, this project focused on understanding the role that mutations in FLT3 play in causing AML and on testing the concept that mutant FLT3 was a valid target for drug therapy. The hypothesis was that inhibition of FLT3 tyrosine kinase activity would be cytotoxic for AML cells and would therefore potentially be of significant therapeutic benefit. We were instrumental in bringing two FLT3 inhibitors to clinical trials, and early phase studies were sufficiently encouraging that at least one of these agents will undergo phase III testing in induction therapy of patients with AML and mutated FLT3 in a cooperative group setting (see project 5). Here, we propose to continue our efforts to understand how to optimally target mutant FLT3, and in addition, propose to initiate specific, focused projects on two other tyrosine kinases mutated in myeloid leukemias, KIT and JAK2. The major focus of the proposal remains on FLT3, The proposed studies are aimed at testing the hypothesis that "combination targeted therapy" for AML has more therapeutic value than use of a kinase inhibitor alone. For example, we predict that targeting both a mutant oncogene, such as FLT3-ITD, and a critical downstream pathway mediating enhanced viability of leukemic cells, such as PI3K, is highly likely to be synergistic. We will also develop higher affinity inhibitors and carefully study resistance mechanisms. If successful, we hope to have a much better understanding of how to design the next generation of FLT3 kinase inhibitor trials in AML. In two other, smaller, specific aims, we propose some focused studies on two other tyrosine kinases that are mutated in either AML (KIT) or Polycythemia Vera (JAK2). These studies will explore therapeutic targeting of these kinases in preclinical models, with the goal of developing clinical trials that can later be conducted in Project 5.